关于被动语态和主动语态.

关于被动语态和主动语态.
主动改被动：
They are doing the project now.
正确是：The project is being done by them now.
为什么要加个being?
Tom has cleaned the room.
正确的是：The room has been cleaned by Tom.
为什么要加个been?
被动改主动：
The house was built in 1990.
正确的是：People built the house in 1990.
为什么不用加个were在built前面?
Things are stolen from the supermaket everyday.
正确的是：People steal thing from the supermarket everyday.
为什么要这么改?
The question is being discussed by a group of students.
正确的是：A group of students is discussing the question.
同上的问题.
请用比较白化的话来回答,英语里的术语看着混乱.
希望回答完以后能够出几道类似的题目让我改改喔,谢谢大家.